I know this is old but it's not cleared. It is not legal to download/copy/distribute any games you don't own. So, it is not legal to just download pirated ROMs or BIOS files and use them in your own way if you don't own the original copy. When it talking about legality, you can only download and play games which have been designed by amateurs (homebrew games) such as from PD ROMs or the ROM Packs website: rompacks dot com. These games are not copyrighted but usually come with bad graphics, most of them are unfinished etc etc. I would suggest you to try to stay legal as much as possible and download only ROMs from games that you already own. For the rest, you can legally download and play any freeware homebrew NDS, PS2 or any other kind of console games.